In the wake of the ongoing deregulations of the electric power markets, load transmission and wheeling of power from distant generators to local consumers has become common practice. As a consequence of the competition between power producing companies and the emerging need to optimize assets, increased amounts of electric power are transmitted through the existing networks, frequently causing congestions due to transmission bottlenecks. Transmission bottlenecks are typically handled by introducing transfer limits on transmission interfaces. This improves system security.
However it also implies that more costly power production has to be connected while less costly production is disconnected from a power grid. Thus, transmission bottlenecks have a substantial cost to the society. If transfer limits are not respected, system security is degraded which may imply disconnection of a large number of customers or even complete blackouts in the event of credible contingencies.
The underlying physical cause of transmission bottlenecks is often related to the dynamics of the power system. A number of dynamic phenomena need to be avoided in order to certify sufficiently secure system operation, such as loss of synchronism, voltage collapse and growing electromechanical oscillations. In this regard, electrical power transmission systems are highly dynamic and require control and feedback to improve performance and increase transfer limits.
With particular reference to unwanted electromechanical oscillations that occur in parts of the power network, they generally have a frequency of less than a few Hz and are considered acceptable as long as they decay fast enough. They are initiated by e.g. normal changes in the system load or switching events in the network possibly following faults, and they are a characteristic of any power system. The above mentioned oscillations are also often called Inter-area modes of oscillation since they are typically caused by a group of machines in one geographical area of the system swinging against a group of machines in another geographical area of the system. Insufficiently damped oscillations may occur when the operating point of the power system is changed, for example, due to a new distribution of power flows following a connection or disconnection of generators, loads and/or transmission lines. In these cases, an increase in the transmitted power of a few MW may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in loss of synchronism, loss of interconnections and ultimately the inability to supply electric power to the customer. Appropriate monitoring and control of the power transmission system can help a network operator to accurately assess power transmission system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed oscillation damping equipment.
There is thus a need for damping such interarea mode oscillations. The conventional way to perform Power Oscillation Damping (POD) is by adding a modulation signal to the control signal of an actuator which counteracts the power oscillation. Typical actuators which could perform POD include synchronous generators, HVDC and FACTS installations. The control system of the actuator is typically implemented in a real-time environment where time delays are small and deterministic. The modulation signal is typically derived from measurements available locally in the substation in which the actuator is installed. The local signals typically include voltage, frequency, line currents and power flows. However, the observability of the inter-area modes of interest may not be sufficiently good in locally available signals. It has therefore been proposed in literature to collect phasors, such as voltage or current phasors, from the different geographical areas. Here two bus voltages, one from each area, may be used. The motivation for this choice would be that these two voltages implement characteristics of two equivalent machines, where each machine represents one of these coherent groups of machines, i.e. one of the geographical areas of the system. In order to dampen the oscillations, phasors from the different geographical areas are therefore collected, for instance using Phasor Measurement Units (PMUs). A PMU typically takes a number of samples, within a specified time interval, from voltage and/or current measurement transformers and calculates positive sequence phasors corresponding to the measurements. The phasors are then time stamped according to an accurate common time reference frame, typically provided through use of the GPS system. The phasors can typically be made available outside the PMU through a communication network using a standard protocol.
However, in order to apply a proper corrective action, the phasors from the two geographical areas need to be aligned in time. This means that a control mechanism needs to operate on phasors that are aligned in time with each other, i.e. have the same time of generation.
Because of this it is common to provide a Phasor Data Concentrator (PDC), which synchronizes the phasors, i.e. packages the phasors with the same time stamp and sends them on to a power control device that performs the damping control.
However, there are a number of problems associated with the above-described damping scheme. The coherent groups are often not very well-defined, in particular when considering that some machines may be out of service at a given point in time, and therefore a selected bus voltage may not be a good representation of a geographical area.
The phase angle of bus voltages may also jump as a consequence of switching events in the vicinity of the bus, contrary to internal machine angles which are associated with inertial (time) constants. A given voltage that is selected to represent a part of the system in a geographical area may therefore be unreliable also because of this.
Furthermore, if one measurement experiences too long a time delay before it is received at a control system of an actuator or is completely lost, the performance of the power oscillation damping algorithm will deteriorate and may even worsen the situation, at least temporarily until data starts to arrive in a timely fashion again.
There is therefore a need for addressing some or all of these problems.